What are the ways to design novel materials for solar energy applications or optimize the existing ones?
A semiconductor suitable for solar cell applications needs to possess two important features. First it has to have a bandgap of around 1.5 eV . Second, the electron-hole pairs that are created as a result of the incident light should have long lifetimes. The aim of my research is to study transition metal oxides and their alloys using ab-initio methods and see how their excited state properties can be manipulated to achieve the aforementioned conditions. Specifically, I work on Nickel Oxide, which is already used in solar applications: it is an effective co-catalyst to produce hydrogen in photocatalysis. Its problem is its large bandgap. Alloying with different materials is a strong candidate to reduce this gap. My work is to use methods such as DFT+U and hybrid DFT to see which one reproduces the ground state properties of pure NiO more accurately. The ground state method is then used to study the effect of alloying on ground state properties and also to optimize the structures that are to be used for excited state calculations. The final aim is to calculate the optical bandgap and carrier lifetimes. The methods used for the latter are Embedded Configuration Interaction and GW. As an improvement on point charge embedding methods that are already used in our group, I am working on including the effects of the polarization of surrounding atoms (which are represented by point charges) using the Shell Model for atomic polarization. Another aim of my project is to calculate conductivity using methods that are being developed by other members of our group. (Back)
How to use orbital-free density functional theory to study liquid metals?
Orbital-free density functional theory (OFDFT) is a powerful linear-scaling method for electronic structure calculations. Instead of calculating individual Kohn-Sham (KS) orbitals, as in KS-DFT, OFDFT uses the electron density as the sole working variable, and thus, is capable of simulating large systems containing millions of atoms. With the recent progress in OFDFT methods, OFDFT is no longer restricted to light metals but can also be applied to semiconductors, transition metals, or molecules. In my study, I will carry out ab-initio molecular dynamics to investigate various properties of liquid metals. These are used as important plasma-facing materials in fusion reactors.The molecular dynamics calculations will be based on newly-developed techniques in OFDFT, such as small-box Fast Fourier Transforms (SBFFT). While traditional FFT implementations scale poorly for more than a few hundred processors, SBFFT enables us to do highly parallelized calculations using tens of thousands of cores to do FFT. The latter will be necessary for large-scale molecular dynamics simulations. (Back)
How to combine CW methods and DFT methods for higher efficiency and accuracy?
A quantitative understanding of a chemical process requires a balanced consideration of efficiency and accuracy. Density functional theory (DFT) scales well with system size, and can readily describe extended systems by the use of periodic boundary conditions, to obtain, e.g., bulk properties. Unfortunately, DFT is limited to ground-state calculations and the use of approximate exchange-correlation functionals. More sophisticated ab-initio correlated wavefunction (CW) methods feature a better description of electronic correlation and excited states. However, the prohibitively large computational cost severely limits their applicability. Furthermore, combining the exact treatment of correlation with the use of periodic boundary conditions is challenging. One possible strategy to overcome these restrictions is to partition a larger system into parts of manageable size that can be treated on different levels of theory: one describes the region of interest (cluster) via a CW method, while the environment is treated with DFT. Such an embedding scheme should reproduce the electronic structure of the cluster while reducing the computational costs to an acceptable level. Our group has developed a potential-functional-based embedding theory [ J. Chem. Phys., 135, 194104 (2011)] for this purpose, that (1) avoids ambiguities by introducing a global embedding potential to model the interaction between subsystems, (2) allows for flexible combination of different methods (DFT, CW), and (3) can be done in a self-consistent fashion. Embedding applications include the interaction of gas molecules with metal surfaces (CO on copper, O2 on Aluminum), that require both a high-level treatment at the adsorption site as well as a description of the extended surface. In the future, the CW/DFT embedding scheme will be used to study the mechanism of a chemical process, e.g., the oxygen reduction reaction on the cathode material of solid oxide fuel cells. (Back)
How to expand the usefulness of reduced scaling multireference configuration interaction methods?
In principle, full Configuration Interaction (CI) provides an exact solution to Schrodinger's equation in a finite basis set. In practice, full CI is too computationally expensive to investigate the chemistry of all but the smallest of molecules. The computational cost can be reduced by truncated CI methods; however these methods do not provide an exact solution. Recently members of the Carter group have proposed a series of methods employing local electron correlation to further reduce the computational cost of CI truncated at the singles and doubles level (SDCI). These local correlation methods remove terms which describe negligible nonlocal electron correlation while retaining the accuracy of the SDCI energy. The goal of our research is continue to expand the usefulness of our local correlation methods. We plan to develop fast algorithms for incorporating size extensivity corrections, calculate transition dipole moments, and energy gradients and further increase the size of molecule we can investigate by parallelizing our computer code. (Back)
How can we convert CO2 into liquid fuels and chemicals?
Identifying renewable energy sources and reducing atmospheric CO2 require immediate attention. Photocatalytic reduction of CO2 to useful products may handle both of these issues. The aim of my research is to study this catalytic process using first principles quantum mechanics methods. In particular I will consider the semiconductor gallium phosphide (GaP) as a photocatalyst. I will investigate the interactions between its surface and the species participating in the CO2 reduction. In addition to the solid catalyst, recent studies suggest that a co-catalyst formed from a pyridinium ion might play a crucial role in achieving efficient CO2 reduction over the semiconductor surface. As part of my research, I will work on elucidating the reaction mechanism involving the pyridinium ion, CO2, water, and the GaP surface that leads to CO2 reduction. (Back)
How can we design catalysts that convert CO2 into liquid fuels?
In my research I am investigating water oxidation and CO2 reduction with the help of a variety of catalysts. We are designing transition metal oxides (TMOs) that exhibit a band gap which is suitable for light absorption in the visible spectrum. The excited electrons can be used to reduce CO2 and the holes can oxidize water. In my research I am modelling all important processes that occur on the surface of the TMOs, e.g. the adsorption of water and CO2 molecules on the surface and cleavage of the C-O bond in CO2 and the O-H bond in water. With our findings we want to understand the mechanisms that govern heterogeneous CO2 reduction and water oxidation and get insight into important properties for the design of new photocatalysts. Complementary to the heterogeneous catalysis by TMOs, a second approach to reduce CO2 uses homogeneous catalysis by a rhenium complex. This complex was experimentally shown to reduce CO2 with high efficiency and selectivity. A similar complex, using much more abundant manganese instead of rhenium, has been shown to exhibit similar catalytic properties, and at the same time working at even less overpotential. In close collaboration with experimentalists we are investigating the redox properties of both complexes and the mechanistic properties of the catalytic reaction. (Back)
How do oxygenated biofuels perform differently from conventional hydrocarbon based fuels?
The typical feature of biofuels which distinguishes them from conventional hydrocarbon fuels is the oxygen atoms included as an additional element in the molecular constitution. The presence of the oxygen atoms in biofuels changes the properties of these fuel molecules, and then makes their combustion behavior different from familiar hydrocarbon fuels. Their chemical decomposition and oxidation pathways are well coupled to the structure of the corresponding fuel molecules. Predicting the combustion behavior of these fuels requires the development of detailed combustion mechanisms, which must include quite a large number of reactions, rate coefficients, as well as related thermochemical and transport parameters. I am using the ab initio multireference configuration interaction method to find out the effect of these oxygen atoms on the fuel molecules, and to improve the accuracy of the parameters which are necessary to define the combustion mechanism. (Back)
Junchao (Steven) Xia
How to extend orbital free density functional theory to more and more broad areas?
Orbital free density functional theory (OFDFT) is an efficient first principles theory that has the potential to provide an effective approach to study large scale material properties. However, it is most reliable only in light metal systems, limiting its applicability to many other interesting scientific phenomena. My current research is to develop and improve OFDFT, particularly for covalent systems. An accurate as well as fast formalism is desired large scale covalent material samples can be studied with OFDFT. Once it is achieved, we can study mechanical or other properties on not only light metals, but also semiconductors or even with molecules with OFDFT with very large numbers of atoms that other first principles theories cannot handle. The ultimate goal is to extend OFDFT to more and more broadly applicable areas and simulate various interesting situations. (Back)